Insulating glass body with electrical feedthroughs and method of preparation

ABSTRACT

A glass wafer having high aspect ratio holes passing therethrough is provided with metal conductors in the holes as feedthroughs. First, the wafer is processed to line the holes with a thin electron-conducting layer of metal. Next, a layer of the same or different metal is applied over the first metal layer providing the requisite feedthrough electrical conductivity for each hole. Preferably, the glass contains lead oxide, the first metal layer is lead and the next layer is copper.

This application is a continuation of application Ser. No. 700,842 filedFeb. 12, 1985 now abandoned which is a division of application Ser. No.576,085 filed Feb. 1, 1984 now U.S. Pat. No. 4,547,832.

This invention relates to the preparation of insulating bodies havingdiscrete pathways therethrough for the passage of electricity, suchdevices being of particular use as high frequency stand-off insulatorsand as part of transparent liquid crystal displays.

This invention is related to inventions disclosed and claimed in theU.S. patent application Ser. No. 204,957--Anthony et al, filed Nov. 7,1980 now U.S. Pat. No. 4,437,109 and U.S. patent application Ser. No.544,935--Anthony et al, filed Oct. 24, 1983 now U.S. Pat. No. 4,570,173.The content of these applications is incorporated herein by reference.

U.S. Pat. No. 3,323,198--Shortes, discloses a method for formingelectrical interconnections through a substrate of semiconductingmaterial using rapid heating with beam of high energy electronsproducing localized dissociation resulting in the formation of anelectrical interconnection passing through the substrate along the holeformed thereby. Whereas the present invention addresses the problem ofproviding electrical interconnections in the form of high-aspect ratioelectrical feedthroughs in a glass body, the Shortes patent is directedto compound semiconductors (Group III-V and Group II-VI compounds) inwhich one of the elements is highly volatile.

A method employing electroforming for the introduction of through-bodyelectrical interconnects for silicon-on-sapphire and semiconductorbodies is described in U.S. Pat. No. 4,396,467--Anthony.

As used herein the term "insulating" or "electrically insulating" glassmeans glass having a resistivity at room temperature of about 10¹¹ohm-cm. or greater.

DESCRIPTION OF THE INVENTION

A body of electrically insulating glass having opposed, spaced majorsurfaces (e.g. a glass wafer) is provided with at least one boreextending completely therethrough. Such bore would have an averagelength-to-diameter ratio of at least 6:1. The glass is to contain atleast about 18 weight percent lead oxide with a maximum lead oxidecontent of about 65 weight percent. By heating the glass body at atemperature of least about 400° C. in a hydrogen atmosphere for asufficient period of time the inner surface of the bore is converted toa thin metallic film of lead. This is accomplished by reducing the leadoxide within a few tenths of a micron of the surface of the bore to leadatoms, which coalesce into the very thin metallic film on the surface ofthe glass. This thin film, although it has a high electrical resistance,will conduct electrons. Thereafter, a layer of metal, preferably copper,is applied over the thin film of lead. It is preferred that the entireapplication of the second layer of metal be accomplished byelectroplating. As an alternative, initially, some of the second metalcan be applied as a thin layer by electroless deposition, but thisshould be followed by completion of the deposition by electroplating.

After completion of the deposition of the second metal, the resultinginsulating glass body having electrical feedthroughs can be used, forexample, to provide through electrical connections for semiconductingchips affixed to the surface of the glass body. This is accomplished byanodically bonding the chips to the surface by applying the appropriatevoltage and then introducing the composite into a solder dip wherebysolder fills the bore to provide front-back through connections from thechip to the opposite side of the glass body.

The diameter of the bore is typically equal to or less than about 10mils and the center line-to-center line spacing between bores istypically less than or equal to about 2 bore diameters. Bore lengthequals the thickness of the glass body, which usually will be a value inthe range of from about 12 to about 500 mils.

BRIEF DESCRIPTION OF THE DRAWING

The features of this invention believed to be novel and unobvious overthe prior art are set forth with particularity in the appended claims.The invention itself, however, as the organization, method of operationand objects and advantages thereof may best be understood by referenceto the following description taken in conjunction with the accompanyingdrawing wherein;

FIG. 1 is a view in section showing a glass wafer having a boreextending completely therethrough interconnecting the major surfaces ofthe wafer;

FIG. 2 is a view of the structure of FIG. 1 wherein the inner surface ofthe bore has been processed to yield a very thin metal layer of leadcovering the inner surface of the bore;

FIG. 3 is a sectional view of the structure shown in FIG. 2 with thesecond layer of metal applied thereto and

FIG. 4 is a sectional view illustrating the utilization of the structureof FIG. 3 as an interconnect to make front-back through connections to achip mounted thereon.

MANNER AND PROCESS OF MAKING AND USING THE INVENTION

FIG. 1 displays a glass body 11 (e.g. a wafer about 25 mils thick)provided with a pair of high aspect ratio bores 12 extending completelytherethrough. The inside surface of each bore 12 is designated as 13 andthe top and bottom surfaces are designated 14 and 16, respectively. Inpractical devices, there will be a large number of bores 12 arranged ina predetermined array or pattern such that correlation may be readilymade between the locations of these bores and the locations at whichconnections are to be made with chips (e.g. silicon chips) to be mountedthereon. These bores preferably are to have an average diameter of lessthan about 1.5 mils and an average length-to-diameter ratio of at leastabout 6:1. Bores 12 can be formed directly in glass plate 11 bylaser-drilling utilizing process limitations to be describedhereinbelow, or by stacking and fusing thin plates of glass withproperly etched surface contours.

The glass employed should have at least about 18 weight percent leadoxide and the typical glass would be a lead-alkali-silicate glass havinga composition range of 30-70 weight percent silicon dioxide, 18-65weight percent lead oxide and 5-20 weight percent sodium oxide (orpotassium oxide). For example, a glass composition of 60% lead oxide,30% silicon dioxide and 10% sodium oxide has the desired high electricalresistivity yet the surface of a body of such glass can be converted toan electrical (not ionic) conductor by heating the body for severalhours at 400° C. in hydrogen gas. As a result of this treatment leadoxide is reduced to a depth of about a few tenths of a micron over thesurface of the glass body. The lead atoms, which result, coalesce into avery thin metallic film over the surface of the glass including theinside of bores 12.

Since the step of heating in hydrogen produces the thin layer of leadover all exposed surfaces of the glass body, provisions must be made forremoving the very thin lead layer from all surfaces except the innersurfaces of the holes. This can be accomplished, after the glass bodyhas been heated in hydrogen, by a affixing the glass body on the vacuumchuck of a photoresist spinner whereby bores 12 are filled with, and thetop of the glass body is covered with, photoresist. Next the photoresistis removed from the top surface 14 of body 11 by exposure to organicsolvents or various commercial mixes (e.g. sulfuric acid/hydrogenperoxide) for removing photoresist. Thereafter, the lead is removed fromall remaining exposed surfaces by a quick dip of body 11 into nitricacid, which removes the lead by etching. Having removed the unwantedlead layer, next the photoresist is removed from bores 12 by a second,longer, exposure to the appropriate solvent or commercial preparation.

A minimum time period of one hour is required at a temperature of400±25° C. to generate a conductive lead film 17. The use of significanthigher temperatures to reduce this annealing time is not beneficial,because of the increased evaporation of lead from the glass body.Annealing at lower temperatures requires too long a time period.

The lead oxide-to-lead transition capability is of particular value inthat lead oxide has a relatively low dissociation energy compared toother glass formers so that the hydrogen reduction can be accomplishedat a relatively low temperature. Further, lead has a relatively lowvapor pressure so that the metallic lead that is formed does notevaporate from the glass surface. In addition, unlike many other glassformers, such as sodium, potassium, or calcium, lead is a relativelyinert material and as a consequence the thin film of lead forming on thesurface of the glass does not react with the ambient air or with theelectroplating solution employed during subsequent processing.Representative glasses containing lead oxide are set forth in TABLE I.

                  TABLE I                                                         ______________________________________                                        Corning                                                                       Glass                                                                         Code No.                                                                             Composition (wt %)                                                     ______________________________________                                        0010   63 SiO.sub.2,                                                                           1 Al.sub.2 O.sub.3,                                                                     7 Na.sub.2 O,                                                                        7 K.sub.2 O,                                                                         22 PbO                               0120   56 SiO.sub.2,                                                                           2 Al.sub.2 O.sub.3,                                                                     4 Na.sub.2 O,                                                                        9 K.sub.2 O,                                                                         29 PbO                               1990   41 SiO.sub.2,                                                                           2 Li.sub.2 O,                                                                           5 Na.sub.2 O,                                                                        12 K.sub.2 O,                                                                        40 PbO                               7570   3 SiO.sub.2,                                                                            11 Al.sub.2 O.sub.3,                                                                    11 B.sub.2 O.sub.3,                                                                         75 PbO                               8161   *40 SiO.sub.2,      5 Na.sub.2 O, 51 PbO                               8160   **56 SiO.sub.2,                                                                         2 Al.sub.2 O.sub.3,                                                                     3 Li.sub.2 O,                                                                        10 Na.sub.2 O,                                                                       23 PbO                               8871   42 SiO.sub.2,                                                                           1 B.sub.2 O.sub.3,                                                                      2 Li.sub.2 O,                                                                        6 Na.sub.2 O,                                                                        49 PbO                               ______________________________________                                         *4 wt % miscellaneous oxides                                                  **6 wt % miscellaneous oxides                                            

Bores, or holes, 12 are the interior longitudinally-extendingcylindrical cavities shown having inner bore surfaces 13. As usedherein, unless otherwise specified, the term "cylindrical" is used todescribed the surface traced by a straight line moving parallel to afixed straight line and intersecting a circle in a plane perpendicularto the fixed line. When introducing holes by laser drilling withlength-to-diameter ratios greater than 20, the walls of the hole tend tohave some taper with the entrance diameter of the hole being slightlylarger than the exit diameter thereof. For purposes of the descriptionherein, however, the axis of each bore is considered to be substantiallyparallel to the inner bore surface 13.

Bores 12 may have an aspect ratio in the range of from about 6:1 toabout 50:1. When these bores are prepared by laser drilling, the aspectratios at the higher end of this range require very sophisticated opticsto enable the careful focusing required. It is expected that as presentequipment is improved and made commercially available, the aspect ratiocan increase still further. Although the laser does not "drill" thebores in the classical sense of the term, that term will be employedherein to describe the preparation of bores by the use of laser beamequipment.

Initial efforts to laser drill glass were conducted at room temperatureand were clearly unsatisfactory due to excessive spalling. It wasconjectured that the problem could be the brittle nature of the glass atroom temperature. When attempts were made to laser drill the glass whilethe glass was heated above its softening point, the holes produced wereclean and free of ragged edges. Thus, in the preferred practice of thisinvention wherein the bores 12 are produced by laser drilling the glassis heated to a temperature at or higher than its softening point andmaintained above the softening point during the laser drilling. TABLE IIprovides data on the softening points for glass compositions in TABLE I.

                  TABLE II                                                        ______________________________________                                        Corning Glass Code#                                                                          Softening Point*                                                                           PbO (wt %)                                        ______________________________________                                        0010           626° C.                                                                             22%                                               0120           630° C.                                                                             29%                                               1990           500° C.                                                                             40%                                               7570           440° C.                                                                             75%                                               8161           600° C.                                                                             51%                                               8160           632° C.                                                                             23%                                               8871           527° C.                                                                             49%                                               ______________________________________                                         *At the softening point, the viscosity of the glass is equal to               10.sup.7.65 poise.                                                       

The drilling may proceed by impinging the laser beam on top surface 14and then proceeding to back surface 16 or, if the glass is transparentto the particular laser, drilling may proceed by impinging the beam onback surface 16 and then drilling to the top surface 14. The latter modeof operation is referred to as reverse laser drilling. The glassesemployed in the practice of this invention are transparent to laserbeams produced by a laser employing a Nd:YAG head.

One laser successfully employed was an ESI, Inc Model 25 Laser ScribingSystem modified with a 10 watt (maximum) optoacoustic Q-switched Nd:YAGhead, manufactured by U.S. Laser Corporation. The laser was operated ina repetitively Q-switched mode with a focus beam size of about 20microns, depth of focus of about 250 microns, an individual pulseduration of about 200 nanoseconds and a repetition rate of about 3 KHz.At a power level of about 2 watts, measured independently in acontinuously pulsed mode, 10 pulse trains of 5 msec duration separatedby a 10 msec delay were used to drill approximately 5 holes per second.If the glass body being drilled is kept at a temperature above itssoftening point, considerably greater latitude can be exercised inselecting the number of pulses to be employed per hole. At successivelylower temperatures, the use of a successively larger number of pulsesbecome necessary in order to avoid ragged edges.

An alternative method of forming bores in a glass body employsphotolithography whereby longitudinally-extending channels are etchedinto the surfaces of thin plates of glass. Thereafter, these thin platesof glass are stacked so that the channels are disposed in opposed pairswith each pair of opposed channels defining a bore. Thereafter the thinplates of glass are fused together and later this unified assembly canbe sliced into wafers with through holes. The fusing process can becarried out either at high temperatures (about 1000° C.) where glasswill fuse naturally or at lower temperatures (about 350° C.) utilizingfield-assisted bonding.

Having carried out the heating in hydrogen of wafer 11 (followed byremoval of the thin lead film from all but the inside surface of bores12), the resulting glass body is shown in FIG. 2 in which the insidesurface of each bore 12 presents a thin layer 17 of lead.

Thereafter, glass plate 11 is placed in a chamber that is firstevacuated and then back-filled with a plating solution that wets layer17 and provides the source for the metal to be deposited byelectroplating over and adhered to lead layer 17. The evacuation isnecessary in order to insure that the plating solution uniformly fillsbores 12 and that no gas bubbles block any of the bores. Cathodiccontact is made to glass plate 11 and a layer 18 of metal selected fromthe group consisting of copper, silver, gold, nickel, platinum and tinis electroplated onto the lead-lined internal surfaces of the throughholes 12.

In the case of using copper as the metal to cover lead layer 17, theaqueous electroplating solution consists essentially of from about 220to about 270 grams/liter of hydrated copper sulfate (CuSO₄.5H₂ O), fromabout 5 to about 28 grams/liter sulphuric acid (H₂ SO₄) from about 0.007to about 0.013 grams/liter of thiourea (N₂ H₄ CS) and from about 0.3 toabout 1.0 gram/liter of molasses. The preferred solution consistsessentially of about 250 grams/liter CuSO₄.5H₂ O, 10 grams/liter H₂ SO₄,0.008 grams/liter N₂ H₄ CS and 0.75 grams/liter molasses. It wasdiscovered that baths of the above composition, of the many differentcompositions tried, resulted in the most satisfactory layers 18 adheredto and covering lead layers 17.

Electroplating solutions applicable in the practice of this inventioncan be found in the text "Principles of Electroplating andElectroforming" by William Blum and George B. Hogaboom [3rd Edition,McGraw-Hill Book Co., N.Y. (1958)].

The uniformity of the thickness of the electroplated film in the hole isenhanced by causing the plating solution to flow through holes 12 duringthe plating operation. This can be accomplished by moving glass wafer,or plate, 11 to and fro in the plating solution or by fixturing plate 11so that a pressure gradient is applied across the plate. Electroplatingis continued until the electrical conductivity of the through holes 12has been made high enough for the specific application.

The minimum thickness of metal layer 18 to be used is related to thediameter of bore 12. This relationship is shown in TABLE III.

                  TABLE III                                                       ______________________________________                                        Hole Diameter (mils)                                                                          Metal Thickness (mils)                                        ______________________________________                                        1               2.5 × 10.sup.-2                                         2               1.3 × 10.sup.-2                                         3               8.5 × 10.sup.-3                                         4               6.4 × 10.sup.-3                                         5               5.1 × 10.sup.-3                                         6               4.2 × 10.sup.-3                                         7               3.6 × 10.sup.-3                                         8               3.2 × 10.sup.-3                                         9               2.8 × 10.sup.-3                                         10              2.5 × 10.sup.-3                                         ______________________________________                                    

It is necessary that metal layer 18 be sufficiently interconnected sothat it forms a continuous electrical circuit between faces 14 and 16 ofbody 11 although complete (i.e. 100%) surface coverage is not requiredunless solder dipping is to be employed for making connectionstherethrough as described hereinafter.

As an alternate to using lead as the metal of layer 17, an initial thinmetal layer of any of the metals recited for use as layer 18 may beintroduced into bore 12 by electroless deposition. When electrolessdeposition is employed for the deposition of the initial very thinlayer, there is no need to limit the composition of the glass to onewhich contains lead oxide. Thereafter, deposition of layer 18 byelectroplating is necessary in order to obtain the necessary thicknessof metal over the inside surface of the hole. Use of electrolessdeposition is limited to bores having aspect ratios in the range of from6:1 to about 10:1. In the case of holes with aspect ratios greater than10:1, it is too difficult to insure the deposition of metal layer 18over the internal surface of the bore along its entire length. Theadvantage, of course, of utilizing a lead oxide-containing glass andcarrying out the lead oxide reduction is that the metal base requiredfor the electroplating step is already at the inner surface of the borewaiting to be reduced by the heating in hydrogen.

Having prepared the structure shown in FIG. 3, its use is exemplifiedschematically in FIG. 4 as further hereinafter explained. Although onlytwo through connections 18 are shown in the device of FIG. 3, typicallythere would be a large number of such small diameter conductive pathsclosely spaced to accommodate a similar large number of regions to whichconnection is required in one or more chips mounted thereon. Chipconstructions such as are described in Ser. No. 204,957 and Ser. No.544,935 illustrate the type of chip, which presents a large number ofthrough connections and, which may need to be accommodated bycomplementary feed throughs in an insulating body as afforded by thisinvention.

Thus, after completion of the application of layer 18 within each bore12, the interconnect of FIG. 4 can be prepared as follows. Chip 21 isproperly positioned and then anodically bonded to wafer 11 (as modifiedin FIG. 3). The anodic bonding is accomplished by the application of avoltage to the temporary assembly. Next, the unified assembly is dippedin molten solder producing solder pins 22, 23 connecting to P junction24 and N junction 26, respectively of chip 21.

What is claimed is:
 1. A process for constructing a device ofelectrically insulative glass having at least one high aspect ratioelectrically conductive path extending therethrough, said processcomprising the steps of:a. etching longitudinally extending channelsinto the opposing surfaces of a plurality of thin plates of glass, saidglass comprising from about 18 to about 65 weight percent lead oxide; b.stacking said glass plates so that the channels are disposed in opposedpairs with each pair of opposed channels defining a bore therein, saidbore having a diameter of less than about 10 mils; c. fusing said platesof glass together to form a single assembly; d. slicing said fusedassembly into wafers with bores having an average length-to-diameterratio of from about 6:1 to about 50:1; e. heating at least one of saidwafers to a temperature of at least about 400° C. in a hydrogenatmosphere for a period of time sufficient to convert the inner surfaceof at least one of said bores to a thin metallic film of lead adhered tosaid glass; and f. adhering a layer of metal selected from the groupconsisting of copper, silver, gold, nickel, platinum and tin to saidfilm of lead, said metal layer providing a continuous electrical pathbetween the major surfaces of at least one of said wafers.
 2. Theprocess of claim 1 wherein said glass is heated to a temperature of400±25° C. during said hydrogen atmosphere heating.
 3. The process ofclaim 1 wherein the bore has a diameter of less than about 1.5 mils. 4.The process of claim 1 in which the fusing of said plates occurs at atemperature of approximately 1,000° C.
 5. The method of claim 1 in whichthe fusing of said plates occurs at a temperature of approximately 350°C.
 6. A process for constructing a device of electrically insulativeglass having at least one high aspect ratio electrically conductive pathextending therethrough, said process comprising the steps of:a. etchinglongitudinally extending channels into the opposing surfaces of aplurality of thin plates of glass, said glass comprising from about 8 toabout 65 weight percent lead oxide; b. stacking said glass plates sothat the channels are disposed in opposed pairs with each pair ofopposed channels defining a bore therein, said bore having a diameter ofless than about 10 mils; c. fusing said plates of glass together to forma single assembly; d. heating said assembly to a temperature of at leastabout 400° C. in a hydrogen atmosphere for a period of time sufficientto convert the inner surface of at least one of said bores to a thinmetallic film of lead adhered to said glass; e. adhering a layer ofmetal selected from the group consisting of copper, silver, gold,nickel, platinum and tin to said film of lead, said metal layerproviding a continuous electrical path between surfaces onto which saidbores open.
 7. The process of claim 6 further including the step of,subsequent to said hydrogen atmosphere heating step, d, or said adheringstep, e, slicing said fused assembly into wafers with bores having anaverage length-to-diameter ratio of from about 6:1 to about 50:1.